The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
In general, unlike a conventional internal combustion engine vehicle (general engine vehicle), a parallel-type hybrid electric vehicle (HEV) satisfies driver's acceleration demand via power distribution of an engine and a motor.
The hybrid electric vehicle is operated to follow an optimal operating line (OOL) to achieve maximum efficiency in order to enhance system efficiency. When driver demand torque is higher than engine torque of the OOL, a difference therebetween is compensated for by motor output (motor driving torque) and when driver demand torque is lower than engine torque of the OOL, a battery is recharged with counter torque (motor regenerative torque) of a motor.
In this case, the demand torque is determined based on vehicle driving information, state information, environment variables, and so on, which are collected from a vehicle by a controller. Then, an operating point with highest system efficiency is determined as an optimal operating point among operating point candidates that satisfy demand torque, and engine operation is controlled according to the optimal operating point.
When demand torque is higher than engine torque according to an OOL, a difference therebetween is compensated for by a motor output to satisfy the demand torque (discharge), whereas, when demand torque is lower than engine torque, a motor is operated as an electric generator by extra output generated by an engine to recharge a battery (charge).
Such a method is a strategy for enhancing engine efficiency with remarkable efficiency change according to an operating point compared with a motor.
As such, when a combination of optimal operating points of an engine and a motor is selected based on system efficiency, an operating point for achieving optimal system efficiency is selected based on engine efficiency and charging efficiency of a motor/battery during charging.
Similarly, an operating point for achieving optimal system efficiency is also selected based on engine efficiency and discharging efficiency of motor/battery during discharging.
This method is a method for achieving optimal system efficiency by considering only charging efficiency for charging and only discharging efficiency for discharging.
However, extra output generated by an engine is not output that is actually transmitted to a power system to generate kinetic energy when only actual charging efficiency is considered but is reconverted into kinetic energy in consideration of both charging efficiency and discharging efficiency.
Similarly, battery output converted into kinetic energy in a motor via discharging of the battery may accurately reflect overall system efficiency in consideration of charging efficiency when the output is first stored in the battery in addition to the consideration of discharging efficiency only.